Today’s motor homes are coming with bigger engines and ever increasing horse power ratings as buyers demand more performance and the ability to tow heavier loads. Probably less advertised or thought of is controlling this mass while descending steep grades. During long downhill runs, the use of the normal service brakes only, can result in excessive heat build up and eventual brake fade, or failure. Fortunately, there are other methods or features that can be used to reduce the use of the service brakes. Let’s look at each.
Large class A gasoline powered coach. Operators gear their automatic transmissions to lower ranges and use the engine as a speed reducer. This works fairly well, as gas engines, with their throttle butterfly, have more resistance than the non restrictive intake of a diesel engine. Additionally, light duty transmission type retarders and after market exhaust brake kits are some of the other methods now being seen. Keep in mind, should you wish to add an exhaust type brake on any vehicle, be sure the transmission is capable of torque converter lock up or similar. There are transmission kits that may be available for your particular model to accomplish this, if needed.
Mid range diesel pusher type A coaches (26k lbs. to 34k lbs.) mostly are equipped with a Pac Brake, similar to the above photo. This was designed and made by Jacobson and is generically called an exhaust brake. The Pac brake consists of a butterfly like valve within the exhaust, that closes when braking is desired. This causes back pressure on the gases that are being forced through the exhaust system and create an engine drag. The engine’s resistance increases with R.P.M., therefore the transmission should be sequentially geared down to maintain the highest engine speed within a safe range. The downshifting is done automatically for you on most, if not all, coaches, and is usually programmed to seek as low as 2nd gear. To activate the Pac Brake, the following must be present. The Pac Brake activation switch must be turned on, the cruise control must be in the non-engaged mode and the throttle position must be at zero (0). Depressing the accelerator pedal while the exhaust brake is operating will cycle the auxiliary brake on and off, which may, in some driving circumstances, be beneficial. There is now an improved version of the popular Pac Brake, the PRXB. This new technology automatically increases the back pressure as the engine RPM’s falls giving it substantially more braking force throughout the RPM range. This is also an upgradable feature on existing Pac Brakes for around $800.
Upper mid to heavy motor coaches are usually equipped with full Jake Brakes. These may be one, two or even three speed brake models. The most common are the one and the two speed, or “Low” and “High”. Unlike an exhaust brake’s method of restricting the exhaust flow, the Jake actually alters the valve train’s internal sequence to emulate a compressor.
A single speed Jake uses all the combustion chambers or cylinders, which on most motor coaches is six. A two speed employs six for “High” and three for “Low”, while a three speed uses six, three and two for “High, Medium and Low” respectively. Jake Brakes work very well and are capable of producing higher stopping horse power than exhaust brake type.
Hydraulic, high horse power transmission retarders are not found on many coaches, but, never the less, are very effective. These would probably only be found on large heavy rigs and conversion type units. The Allison hydraulic retarder, for example, basically uses a vaned flywheel within the transmission housing. Oil is directed into the vanes which absorb the vehicles energy and converts it to heat. The engine’s cooling system then disperses the heat. This is generally the most expensive and effective method for supliment braking.
Compression type brakes are a very important feature, particularly on diesel pushers. The heavy vehicle weight causes the speed to increase at a rapid rate if not checked on steep down hill grades. The service, or foundation brakes, are not capable of being dragged or steadily used during long descents without creating excessive heat. This heat will eventually cause brake fade and may lead to total brake failure. Therefore it is important to know the braking features of your vehicle and how to best employ them. Always refer to the owner/operator manual for proper use and maintenance.
Next week, we will be looking at practical applications and using compression type braking. Also we will have some useful tips, and as usual, we hope to hear from you with your comments.
Until Then Lug_Nut
Willie Combs
I have a few comments on braking on steep grades, I am a retired Truck Driver and have dealt with this problem many many times, with heavy loads the key to this is to not let the unit get fast to begain with, this means start slow and keep it that way, keep lite presser on the brake don’t take your foot off this will only let the unit speed up and make it harder to slow back down. Gearing down will help so if you do both you should not have any trouble.
bob
Has anyone seen the Drive shaft magnetic brake unit for gas rigs? Saw one last year on a 37foot class A and seems like it is good. Made in Oregon, wraps around the drive line and used 12v batterys to stop the shaft from turning, could set the DC volts from 1-12 for more or less break. Steady, no heat to speak of no friction?? sounds like a good thing. Thats all I know about it, cost? where you can get it ? and install? weight? bob
Lug_Nut
Casey, Thank you for picking up on that, I have made the change. The cylinder temperature produced by the actions of the Jake Brake is about 900 F. So in a sense you may be sacrificing heat to reduce the speed of the momentum. 900 degrees F. would certainly burn and destroy some materials.
Casey Donovan
I hate to be a pedant, but a professional writer should know the difference between the words he uses and similar sounding words.
I really don’t want to “immolate a compressor”, especially if it’s one I own. That means to sacrifice it, usually to burn it up, literally. Now, if my diesel engine with a Jake brake can “emulate” one, that may be helpful.
Gypsy John
Having had vehicles with exhaust brakes, Jake Brakes and now the Allison Retarder–I will NEVER go back to anything that doesn’t have the Allison Transmission Retarder. I have a “joystick” with seven positions and rarely use my service brakes unless caught in an emergency situation. I have crossed over the Rocky mountains going down 10% grades for six continuous miles with all three braking systems and absolutely scared to death with the exhaust brake system and just slightly less scared with the Jake brake. Retarder was a total piece of cake. Traveled down Tehacapi & the Grapevine in CA–again no fear or problems with the Retarder. I can’t say the same for the other kinds of engine braking systems.
I’ve driven big rig tractor trailer commercial vehicles, 54 passenger buses, and several motorhomes so I have well rounded level of experience with just a over one million miles under my belt.
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